VS8 failed to convert Video file - NTSC

Guest

I have a number of projects which I have successfully created Image files for in PAL format, when I change the format to NTSC during the conversion process the following error message comes up. "Failed to convert video file Error: 80041c21 ....." It appears to occur when converting one of the large clips as if I move this clip earlier in the series the erro message occurs earlier in the conversion.
Has anyone else encountered this problem?